The present invention relates to semiconductor wafer fabricating equipment; and more particularly to apparatus for rinsing particles from semiconductor wafers during fabrication.
Semiconductor fabrication utilizes large quantities of water for rinsing the semiconductor wafers at different stages of fabrication, as well as for rinsing the equipment that processes the wafers. For example, the semiconductor wafer must be rinsed with water following polishing to flush away particles of the abrasive slurry which adheres to the wafer. In addition, the polishing pad is rinsed between operations to remove accumulated slurry and wafer particles which adhere to the pad so that those materials will not affect polishing of the next semiconductor wafer. Typical chemical mechanical planarization (CMP) equipment can consume two to eight gallons of water per minute. This equates to between 50 and 200 gallons of water per wafer depending on the number and type of operations being performed on each wafer.
Heretofore, such semiconductor wafer processing equipment was supplied with ultra pure, deionized water with a resistivity of about 18 mega-ohms, for example. After rinsing one semiconductor wafer or piece of the processing equipment, the water, now contaminated with waste particles and process chemicals, was discarded through a drain. The accepted principle is that if the waste water is recycled, particles of slurry and wafer material born by the waste water could damage the subsequently rinsed semiconductor wafers. Concern also existed that after a rinsing operation the waste water no longer has a neutral pH which also would adversely affect semiconductor wafers processed subsequently.
For example, silica CMP employs an alkaline abrasive slurry having a pH of 10.5-11.5 at working concentration. This slurry contains up to 10-12 percent by weight of silica as microscopic abrasive particles and ammonium, sodium, or potassium ions equivalent to about a 0.5-2.0 percent by weight solution of their respective hydroxide. In addition the slurry typically has several hundred parts per million of soluble silica as silicate. When the silica CMP processing solutions are diluted by the rinsing operation, the waste water may have a pH of about 9 and the dissolved silica begins to decompose forming insoluble silicon oxide particles.
The slurry used in metals CMP normally is acidic. Such a slurry that is widely used to remove tungsten consists of a dilute ferric nitrate solution having 0.5 to 3.0 percent by weight of microscopic, abrasive aluminum oxide (alumina) particles. Other types of abrasive solutions used in metals CMP include (a) ferrocyanide, organic acid and silica or alumina; (b) hydrogen peroxide and alumina; and (c) potassium periodate and alumina. When metals CMP processing solutions are diluted by the rinse operation, the pH changes due to dilution. Any alumina from the processing is essentially insoluble; whereas ferric nitrate from the processing hydrolyzes at a pH greater than 3 to give ferric hydroxide solids, free nitric acid and unhydrolyzed ferric nitrate. Ferric nitrate hydrolysis will reach an equilibrium with a concentration of free nitric acid which is sufficient to redissolve the ferric hydroxide precipitate. At this point the weakly acidic (pH 3-6) solution is stable and will keep the remaining ferric nitrate in solution.
Additional concerns have been expressed that ionic contamination will increase to unacceptable levels in a closed rinse water recycling system and adversely affect removal of particles from semiconductor wafers. Hence rinse water from standard chemical mechanical planarization (CMP) processing equipment was discarded after one rinse operation.
Therefore, conventional wisdom regarding semiconductor fabrication dictated that only ultra pure, deionized water be used so as not to contaminate or damage the semiconductor wafers. As a consequence it was not considered practical to recycle the waste water to rinse more than one semiconductor wafer. This consumption of water is not only wasteful, but the deionized water is expensive.
Thus previous approaches to re-using sent the rinse water back through a standard ultra-pure water processing system to regenerate 18 mega-ohm water with all particles removed. There are two options for repurifying the rinse water. One is to send filtered used rinse water to the main processing plant for the ultra pure water. However, this option is not recommended by many semiconductor manufacturers for fear of introducing undesirable impurities from the CMP operation into the entire purification plant. A more acceptable option is to use a separate ultra pure water purification system which is dedicated to processing the used rinse water. The water from this second purification system is used only back in the CMP equipment.
Both of these options are very costly to implement, with high capital and operating expenses. Furthermore due to the nature of the ultra pure water purification, a 25% to 40% of the incoming waste rinse water must be discarded.